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Cruz et al. (2018). “THM densification of wood,” BioResources 13(2), 2268-2282. 2268

Impact of the Chemical Composition of Pinus radiata Wood on its Physical and Mechanical Properties Following Thermo-Hygromechanical Densification

Noemi Cruz,a Cecilia Bustos,a,b,c,* María Graciela Aguayo,b Alain Cloutier,c and

Rosario Castillo d

The thermo-hygromechanical densification process changes the chemical composition and the physical and mechanical properties of wood. The aim of this work was to study the impact of the chemical composition of Pinus radiata wood on its physical and mechanical properties following the thermo-hygromechanical densification process. The samples were initially segregated by lignin content. Density, hardness, modulus of elasticity (MOE), and modulus of rupture (MOR), in addition to lignin, α-cellulose, hemicellulose, and extractive contents, were determined before and after the densification process. The results indicated that densified wood with high initial lignin content had greater rate of increases in density and MOE than wood with lower initial lignin content. Additionally, densified wood with lower initial lignin content had greater rate of increases in hardness. The rate of increase of MOR did not show significant differences within both groups. Carbohydrates present in the control and the densified wood played an important role in the mechanical strength of the final product.

Keywords: Pinus radiata; Thermo-hygromechanical process; Wood densification; Chemical components

Contact information: a: Departamento de Ingeniería en Maderas, Universidad del Bío-Bío, Concepción,

Chile; b: Centro de Biomateriales y Nanotecnología (CBN), Universidad del Bío-Bío, Concepción, Chile;

c: Centre de recherche sur les matériaux renouvelables (CRMR), Université Laval, Quebec City, QC,

Canada; d: Departamento de Análisis Instrumental, Facultad de Farmacia y Centro de Biotecnología,

Universidad de Concepción, Concepción, Chile; *Corresponding author: [email protected]

INTRODUCTION

Densification is a modification method in which the wood cell structure is

compressed in a hot press to obtain a higher-density material (Laine et al. 2014). The

natural elastic properties of the wood, as well as its density, moisture content, orientation

of the sample, and compression ratio play important roles in the densification process

(Kutnar and Šernek 2007; Pelit et al. 2014). This technique yields a reduction in the void

volume of the wood, which increases its density and enhances its mechanical properties

(Navi and Heger 2004; Fang et al. 2012a; Chávez 2015).

The thermo-hygromechanical (THM) densification process can be performed in

an open system by compression combined with steam injection (Navi and Sandberg

2012). THM densification reduces wood hygroscopicity, improves dimensional stability,

enhances resistance to microorganisms, and releases internal stresses (Fang et al. 2011;

Diouf et al. 2011; Navi and Sandberg 2012; Rautkari et al. 2013; Chávez 2015). Some

wood densification studies have shown increases in density and Brinell hardness of 100%

or more (Kamke 2006; Fang et al. 2011; Bustos et al. 2012). The modulus of elasticity



(MOE) and modulus of rupture (MOR) in static bending of THM densified wood

increase by 50% and 100%, respectively (Mariotti 2010; Fang et al. 2011; Chávez 2015).

The chemical composition of wood changes when it is subjected to modification

processes involving steam, heat, and mechanical compression (Navi 2011; Kariz et al.

2017). The THM densification process in softwood increases the Klason lignin content,

and extractive substances are evaporated or decomposed (Boonstra et al. 2007; Rousset et

al. 2009; Liu et al. 2014). Fourier-transform infrared spectroscopy (FTIR) analyses in

densified softwoods show a reduction in the degree of polymerization of hemicelluloses

and lignin condensation reactions. This results in an increase of lignin-carbohydrate

complexes, increased crystalline cellulose content, glucomannan decomposition, and loss

of C=O groups bound to the lignin backbone (Yin et al. 2011; Guo et al. 2015).

Although there are studies that report the effects of the densification process on

the physical and mechanical properties of wood and its chemical composition (Diouf et

al. 2011; Sandberg et al. 2013), there is little information on how the chemical

components of wood are related to its physical and mechanical properties, especially

those involved in the THM process. Therefore, the objective of this work was to study the

impact of the chemical composition of wood on the physical and mechanical properties

of Pinus radiata after THM densification.

EXPERIMENTAL Materials

For this study, 28 commercial planed sawn boards of Pinus radiata provided by

CMPC Maderas sawmill (Los Ángeles, Chile) were used. Their dimensions were 7 mm ×

80 mm × 1000 mm. Their average moisture content (dry basis) and density (at 9%

moisture content) were 9% and 500 kg/m3, respectively. Each board was graded visually

to obtain pieces free of visible defects such as knots, cracks, and bark pockets.

Additionally, only sapwood was considered within the boards. A ferric chloride stain was

used to discard the heartwood. The boards were cut into two parts: a control sample and a

sample to be used for densification. The 28 control samples and 28 densified samples

were characterized in terms of chemical composition, physical properties, and mechanical

properties as depicted in Fig. 1.

Fig. 1. Schematic representation of the distribution of test samples for measurement of density, color, bending properties, hardness, and chemical composition of control and densified wood



Determination of Chemical Composition The chemical composition was determined in the control and densified wood.

Both control and densified wood were milled in a knife mill (FRITSCH, Germany) and

sieved to 45 to 60 mesh. To determine the amount of extractives, the milled wood was

extracted in a Soxhlet apparatus with acetone for 16 h according to TAPPI T280 pm-99

(2015). Extractive-free milled wood was used in the chemical characterization assays.

The holocellulose and α-cellulose were measured according to the TAPPI T 9 m-

54 (2015). Holocellulose was prepared with 250 mg of extractive free milled wood and

then adding 5 mL of deionised water, 2 mL CH3COOH, and 5 mL of 80% NaClO2. The

samples were immersed in a water bath at 90 °C for 1 h. After this period, an additional 4

mL CH3COOH and 10 mL of 80% NaClO2 were added to the flask, and the reaction was

carried out for another 1 h at 90 °C. The reaction was stopped by immersing the flask in a

water bath at 10 °C. Solids were filtered and washed with deionised water, and dried at

105 °C to constant weight, with the remaining weight quantified as holocellulose. Later,

100 mg of holocellulose was treated with 8 mL of 17.5% (w/v) NaOH for 30 min at room

temperature with stirring every 10 min. Next, 8 mL of distilled water was added to the

solution, and the reaction was carried out for an additional 30 min. Solids were filtered,

washed with of distilled water, and impregnated with 20 mL of 1.0 M CH3COOH for 5

min. The residue was washed with excess water, dried at 105 °C to constant weight, and

quantified as α-cellulose. All analyses, holocellulose and α-cellulose, were performed in

triplicate.

The lignin content was determined as described by Aguayo et al. (2010). Samples

of 300 mg were weighed in a test tube, and 3 mL of 72% (w/w) H2SO4 was added.

Hydrolysis was performed in a water bath at 30 °C for 1 h with stirring applied every 10

min. Subsequently, the acid was diluted to 4% (w/w) and transferred to a 250 mL

Erlenmeyer flask and autoclaved for 1 h at 121 °C. The residual material was cooled and

filtered. Solids were dried to constant weight at 105 °C and classified as insoluble lignin.

Soluble lignin was determined by measuring the solution absorbance at 205 nm (Dence

1992). Total lignin was calculated as the sum of insoluble and soluble lignin. Analyses

were performed in triplicate for each sample.

Segregation of Samples

Lignin is a native plasticizer embedded in the cell walls. If all the lignin in the cell

walls could be softened, then the wood would exhibit plasticizing during its modification

(Wang et al. 2011). To analyze the behavior and effects of the chemical composition in

Pinus radiata, the samples were segregated by lignin content in control wood (C). Two

groups were identified: group A, with low lignin content, and group B, with high lignin

content. After undergoing the THM densification process, both groups were identified

with the letter D.

Densification Process

Planed sawn boards of 7 mm × 80 mm × 500 mm were densified using a thermo-

hygromechanical (THM) process. The densification process was carried out at the

Renewable Materials Research Center (Centre de recherche sur les matériaux

renouvelables, CRMR), Université Laval, Quebec City, Quebec, Canada. A steam

injection press (Dieffenbacher, Alpharetta, GA, USA) with dimensions of 862 mm × 862

mm was used for the densification treatment (Fang et al. 2012b). Steam injection holes



with a diameter of 1.5 mm were distributed uniformly at 32 mm intervals on both the

upper and lower platens of the press. The two platens were preheated to the target

temperature before treatment. Steam was continuously injected throughout the

densification process at a maximum pressure of 550 kPa. The Pinus radiata boards were

densified at 180 ºC and 9000 kPa of pressure with 550 kPa of steam pressure for 7 min

according to Chávez (2015), providing a densification ratio of 50%. The press was kept

closed for 5 min to prevent an increase in thickness of the samples due to the springback

effect. After the THM process, the boards were conditioned at 20 ºC and 65% relative

humidity until constant weight.

Determination of Physical and Mechanical Properties

Density

The samples were placed in a conditioning room at 20 ºC and 65% relative

humidity to reach an equilibrium moisture content of 12%. The dimensions of the control

samples were 7 mm × 50 mm × 50 mm and the dimensions of the densified samples were

3.5 mm × 50 mm × 50 mm. Following the conditioning stage, the samples were measured

and weighed for apparent density determination (kg/m3) according to ASTM D1037

(2012).

Static bending

Three-point static bending tests were performed according to ASTM D1037-12

(2012) on densified and control samples with a Zwick Roell Z020 Universal Testing

Machine, with a capacity of 20 kN. The span was 24 times the nominal thickness of the

samples. The load was applied continuously throughout the test at a uniform rate. The

modulus of rupture (MOR) and the modulus of elasticity (MOE) were determined with

Eqs. 1 and 2, respectively,

(1)

(2)

where b is the width of the sample (mm), d is the thickness of the sample (mm), L is the

span (mm), P is the maximum load (N), ΔP is the loading increment in the proportional

section of the load–deflection diagram (elastic zone) (N), and Δδ is the deflection

measured at mid-span (mm).

Brinell hardness

Brinell hardness tests were conducted using a universal testing machine according

to the European standard EN 1534 (2000). The testing machine was equipped with a

hardened steel ball indenter with a diameter of 10 mm. A maximum load of 1000 N was

reached in 15 s and maintained for 25 s. The load was then released. The measurement of

the diameters of the indentations were made with a digital caliper with ± 0.01 mm

sensitivity and a magnifier. The Brinell hardness HB (N/mm2) was calculated using Eq.

3,



(3)

where P is the maximum load (N), D is the diameter of the steel ball indenter, and d is the

average diameter of the indentation (mm).

Color analyses

In order to evaluate color differences of densified wooden surfaces, color

measurements on the control and densified samples were carried out. Color

measurements were conducted by a portable color spectrometer (spectro-guide sphere

gloss meter, model CD-6834, BYK-Gardner GmbH, Geretsried, Germany). The

colorimeter was calibrated at D65 lighting and observing angle of 10º. The CIE L*a*b*

chromaticity system was used for color measurements. The color is described by 3

coordinates: L* represents the lightness, a* represents the content from red to green, and

b* represents the content from yellow to blue. According the CIE L*a*b* color system,

the total color difference was determined using Eq. 4 (Mitani and Barboutis 2014) from

two measurements taken for each piece:

(4)

where ΔL*, Δa*, and Δb* represent the differences between the tested sample and the

standard piece. These quantities can be calculated by the difference between the

coordinate value of the sample and the coordinate value of the control. The total color

change ΔE* represents the color difference.

FTIR spectroscopy

The control and densified wood samples were analyzed by FTIR spectroscopy.

The spectra were acquired in transmittance mode using the KBr technique for sample

preparation. The spectra range was between 4000 and 400 cm-1 with a resolution of 4 cm-

1 and 32 scans, using a Spectrum BX FTIR spectrometer (PerkinElmer Inc., Waltham,

MA, USA). A spectrum was obtained for each control and densified sample. The spectra

were normalized in the 1030 cm-1 band and averaged by groups for analysis.

Statistical Analysis

The normality (Shapiro-Wilk) of the physical and mechanical properties and

chemical composition data distribution, ANOVA, Tukey test (Tables 1 and 2) and a t-test

(Table 3) were performed using the software Statgraphics Centurion XVII (Statpoint

Technologies, Inc., The Plains, VA, USA). The principal component analysis (PCA) was

performed using all the physical and mechanical properties. The chemical composition

data were obtained in order to evaluate the spontaneous separation of samples, using the

software Pirouette 4.5 (Infometrix, Inc., Bothell, WA, USA) after a scaling of the data.

Correlations between chemical composition and physical and mechanical properties were

determined using Pearson’s coefficient.



RESULTS AND DISCUSSION

Chemical Composition of Control and Densified Wood Table 1 shows the chemical composition of the P. radiata samples analyzed, from

the two groups of segregated control wood (C) and densified wood (D).

Group AC (14 pieces) presented low lignin content values (from 25.2% to

27.3%). In contrast, group BC (14 pieces) contained samples with higher lignin content

(from 27.6% to 32.7%). These control groups presented significant differences at p <

0.05 (Tukey test). After the densification process, group AD (14 pieces) presented lignin

content values from 31.6% to 36.8%, and group BD (14 pieces) presented lignin content

values from 32.1% to 47.6%. It is known that, after densification, the wood presents

changes in its chemical composition (Kutnar and Šernek 2007; Diouf et al. 2011; Neyses

et al. 2016; Yin et al. 2017). In this study, it was observed that lignin content increased

for both groups (low lignin content and high lignin content). This may be due to a partial

depolymerization of lignin and a rearrangement of the hemicelluloses (Navi and Pizzi

2015), as the hemicelluloses easily degrade due to their amorphous structure and low

molecular weight (González-Peña et al. 2009). Degradation of hemicelluloses in the

densification process has been reported in previous works (Tjeerdsma and Militz 2005;

Boonstra 2008). Similar results were obtained in this study, where average hemicellulose

contents were 28.9% and 29.5% (groups AC and BC, respectively). These numbers

decreased drastically following the densification process (5.5% and 7.6% for groups AD

and BD, respectively).

The average α-cellulose content in control groups AC and BC was 41.7% and

38.4%, respectively. After densification, the α-cellulose increased to 53.4% and 50.7%

for groups AD and BD, respectively. There was a significant difference in mean

extractive content between groups AC and BD, as can be seen in Table 1. The extractive

content may increase due to the degradation of hemicelluloses and lignin, which may

generate new extractives (Liu et al. 2014; Li et al. 2016).

Table 1. Chemical Composition of Densified and Control Samples of P. radiata

Component

AC BC AD BD p-value

Lignin (%) Range 25.2-27.3 27.6-32.7 31.6-36.8 32.1-47.6

< 0.0001 x̄ 26.5 d 29.3 c 34.2 b 37.7 a

Holocellulose (%) Range 66.8-73.6 62.5-72.1 55.8-63.8 52.7-65.3

< 0.0001 x̄ 70.6 a 67.9 b 58.9 c 58.3 c

α-Cellulose (%) Range 38.6-45.0 34.6-42.4 46.3-62.9 40.9-70.0

< 0.0001 x̄ 41.7 c 38.4 d 53.4 a 50.7 b

Hemicelluloses * (%)

Range 26.2-31.8 26.4-32.0 0-16.0 4.7-14.7 < 0.0001

x̄ 28.9 a 29.5 a 5.5 c 7.6 b

Extractives (%) Range 0.56-3.11 0.26-4.29 1.56-3.53 0.40-4.27

0.0107 x̄ 1.84 b 2.11 ab 2.20 ab 2.34 a

* Hemicellulose content was calculated as holocellulose content − α-cellulose content (Li et al. 2016). Values within a row with different letters present a significant difference at p < 0.05 (Tukey test). AC: control samples with low lignin content; BC: control samples with high lignin content; AD: densified samples with low lignin content; BD: densified samples with high lignin content



Physical and Mechanical Characterization Table 2 shows the results obtained for the physical and mechanical properties of

the samples from the groups low in lignin content and high in lignin content. Following

densification, the samples’ density increased from 546 kg/m3 to 1163 kg/m3 for group A

and from 506 kg/m3 to 1101 kg/m3 for group B. This can be explained by the decrease in

the volume of cavities and the increase in the amount of cell wall per unit volume

according to the compression ratio of 50% (Navi and Sanberg 2012; Pelit et al. 2015).

The Brinell hardness also showed a considerable increase, from 16 to 97 MPa for group

A and 17 to 96 MPa for group B. In the case of bending, the MOE values of densified

wood were 18100 MPa and 17300 MPa for groups A and B, respectively, while the MOR

presented values of 216 and 192 MPa for groups A and B, respectively. Similar results

were found by Kutnar et al. (2009), Fang et al. (2012a), and Chávez (2015).

The total color change was observed by a decrease in L*, indicating a loss of

lightness (reflectance, evaluated in the green wavelengths), possibly due to a degradation

of the hemicelluloses in thermally treated wood and therefore an increase in lignin

content (Huang et al. 2012; Bekhta et al. 2014). For the value of a*, the samples had a

more reddish color in group B. This color can be associated with the extractive content

of the wood (Gierlinger et al. 2004). The values of b* increased, with the wood

becoming more yellow. This color results from the photochemistry of the basic

components of wood, particularly lignin (Pelit 2016). The color change (ΔE) showed no

significant difference between the two groups of densified wood.

Table 2. Physical and Mechanical Properties of Control and Densified Wood

Property

AC BC AD BD p-value

Density (kg/m3)

Range 473-624 394-574 1066-1308 919-1334 < 0.0001

x̄ 546 c 506 c 1163 a 1101 b

Hardness (MPa)

Range 13-19 10-25 76-132 60-136 < 0.0001

x̄ 16 b 17 b 97 a 96 a

MOE (MPa)

Range 10100-17600 5710-15800 16000-20500 6700-25800 < 0.0001

x̄ 14010 b 12131 b 18091 a 17335 a

MOR (MPa)

Range 92-136 62-109 190 -243 97- 270 < 0.0001

x̄ 108 c 89 c 216 a 192 b

L* Range 70-82 74-82 67-78 69-78

< 0.0001 x̄ 76 a 78 a 73 b 74 b

a* Range 4-9 4-8 9- 10 6-8

0.0002 x̄ 6.5 b 6 b 7 a 7 ab

b* Range 23-30 22-29 24-31 24-31

< 0.0001 x̄ 25 b 24 b 28 a 27 a

ΔE Range - - 1-8.6 2-10

0.4608 x̄ - - 4.8 a 5.5 a

Values within a row with different letters present a significant difference at p < 0.05 (Tukey test). AC: control samples with low lignin content; BC: control samples with high lignin content; AD: densified samples with low lignin content; BD: densified samples with high lignin content



FTIR Analysis FTIR spectra of control and densified wood were analyzed (Fig. 2). Band

assignments were made according to Pandey (1999) and Faix (1992). Figure 2 shows the

spectra of groups A and B for control and densified wood. Small differences were found

among the spectra, which is most likely a consequence of the short processing time

(Kutnar et al. 2008).

The analysis was focused on characteristic bands of the infrared spectra of the

control and densified wood in groups A and B: 1740 cm-1 (C=O stretch in unconjugated

ketones), hemicellulose characteristic band; 1640 cm-1 (water associated with lignin or

cellulose); 1510 cm-1 (C=C stretching of the aromatic ring (with G unit of lignin)); 1458

cm-1 (C−H deformation in –OCH3 in lignin and CH2 symmetric scissoring in pyran ring),

and 1269 cm-1 (C−H of guaiacyl ring in lignin), characteristics of lignin. The bands 1374

cm-1 and 1169 cm-1 (CH bending in cellulose I and cellulose II and hemicellulose and

C−O−C asymmetric stretching in cellulose I and cellulose II), which characterize the

carbohydrates, were also considered.

Fig. 2. FTIR spectra of control and densified wood. AC: control samples with low lignin content; BC: control samples with high lignin content; AD: densified samples with low lignin content; BD: densified samples with high lignin content

Effect of Chemical and Physical-Mechanical Features in THM Process

In the THM densification of wood, factors such as temperature and pressure affect

the wood’s chemical components (individually and their interactions) and therefore its

mechanical properties (Boonstra et al. 2007). All the results obtained (chemical

composition and physical and mechanical properties) were also analyzed by PCA, which

is a multivariate data analysis method suitable for describing major trends in a data set

and the relationships among samples and among variables (Aguayo et al. 2010).

Figure 3A shows the PCA results of control and densified samples. The PCA

identified patterns to highlight their similarities and differences. The results show that the



first component accounted for 73.8% of the variance of the data. The main separation of

the groups was defined by PC1 and PC2, obtained following the process of densification.

The more relevant components for this separation were lignin, hemicellulose, α-cellulose,

density, and MOE (Fig. 3B). Hardness and MOR mainly characterized the densified

samples, while the control was characterized by MOE and chemical composition (lignin,

α-cellulose, hemicellulose, and holocellulose). Figure 3C shows the PCA of the control

and densified samples of group A, in which the first main component explained 84.8% of

the variance of the data. The main separation of the data was defined by PC1 and PC2,

and the components that most influenced this separation were lignin, α-cellulose,

hemicellulose, MOE, color, and density (Fig. 3D). Density mainly characterized group

AD, while group AC was characterized by lignin, α-cellulose, hemicellulose, MOE, and

color. Figure 3E shows the PCA of the control and densified samples of group B, in

which the first main component explained 73.7% of the variance of the data. The main

separation of the data was defined by PC1 and PC2, and the components that most

influenced this separation were lignin, α-cellulose, hemicellulose, density, MOE,

extractives, and color (Fig. 3F). Holocellulose, hemicellulose, and L* characterized group

BC, while group BD was characterized by hardness, MOR, extractives, and a*.

This information was in agreement with the data for the control samples (Tables 1

and 2), where there were significant differences between groups A and B in chemical

composition but not in physical and mechanical properties. In contrast, the densified

wood showed differences in chemical composition and also in some physical and

mechanical properties.

The main chemical components of the cell wall contribute to different degrees to

the mechanical properties of wood (Winandy and Rowell 2005). With the segregation of

the samples by lignin content, it was possible to establish groups that could provide

information about the behavior of the wood’s chemical components during THM

densification and to analyze whether segregation affected the various properties of the

final product. Table 3 shows the increases in density, hardness, and MOE in densified

wood by group, with significant differences in the properties studied. Density increased

by 118% in group B but by 109% in group A.

When grouping all the data of density and lignin content, a correlation coefficient

of r = 0.9 was observed for group A and r = 0.7 for group B, with a greater dispersion of

data for group B (Fig. 4A). Hardness increased by 512% for group A and 415% for group

B, compared to the control samples. High correlation coefficients between hardness and

lignin content were seen in both groups, with r = 0.8 for group A and r = 0.7 for group B

(Fig. 4B).

This strong relationship may occur because the hardness of the cell wall is

dominated by the matrix composition (Gindl et al. 2004). In the densification process, a

reduction in hemicellulose content has been reported (Li et al. 2016). This chemical

component of the wood is relevant because the main role of the hemicelluloses is to act as

a bonding material between the microfibrils, allowing their mobility in the deformation

process (Keckes et al. 2003). This information may explain this study’s results for MOE,

where the higher increase, 57%, was obtained for group B. This can be related to the

residual hemicellulose content in the densified wood of group B (7.6%), while group A

had a lower MOE increase, 28%, and a lower hemicellulose content in its densified wood

(5.5%) (Tables 1 and 3).

MOE and hemicellulose showed high correlation in both groups (Fig. 4C). MOR

increased by 104% for group A and 106% for group B: a statistically insignificant



difference between the groups. Figure 4D shows a high negative correlation between

MOR and hemicellulose content in both groups (r = -0.935 and r = -0.840, for groups A

and B respectively).

Fig. 3. Principal component analysis of chemical composition and physical and mechanical properties. A: scores of control and densified wood; B: loadings of control and densified wood; C: scores of control wood; D: loadings of control wood; E: scores of densified wood; F: loadings of densified wood. AC: control samples with low lignin content; BC: control samples with high lignin content; AD: densified samples with low lignin content; BD: densified samples with high lignin content

A

FE

DC

B



Table 3. Rate of Increase in Physical and Mechanical Properties of Groups A and B

Property Increase in Densified Wood by Group (%)

p-value A B

Density 109 b 118 a 0.0041

Hardness 512 a 415 b 0.0473

MOE 28 b 57 a 0.0018

MOR 104 a 106 a 0.9001

Values within a row with different letters differ significantly at p < 0.05 (t-test).

Fig. 4. Relationships between chemical composition and physical and mechanical properties of control and densified woods. A: density vs. lignin content; B: hardness vs. lignin content; C: MOE vs. hemicellulose content; D: MOR vs. hemicellulose content. AC: control samples with low lignin content; BC: control samples with high lignin content; AD: densified samples with low lignin content; BD: densified samples with high lignin content

CONCLUSIONS 1. The wood samples of high initial lignin content increased more in density and MOE

after the densification process than did the samples of low initial lignin content.

However, the rate increase of hardness after THM densification was greater for wood

of low initial lignin content than for wood of high initial lignin content.

2. The carbohydrates (α-cellulose and hemicelluloses) present in the control and

densified samples play a major role in determining the mechanical strengths of the

final products, contributing significantly in MOE and density.



3. The principal component analysis indicated that lignin content is not the only

important factor in the separation of the groups; hemicellulose and α-cellulose

contents are important, too. In contrast, the physical and mechanical properties that

contribute to the separation of groups were density and MOE.

ACKNOWLEDGMENTS

The authors wish to thank the Renewable Materials Research Center (Centre de

recherche sur les matériaux renouvelables) of Université Laval, Quebec City, Quebec,

Canada, and the Centro de Biomateriales y Nanotecnología at the Universidad del Bío-

Bío, Concepción, Chile, for logistical support and equipment testing. The authors also

thank CMPC Maderas for providing Pinus radiata boards and the MINEDUC-UBB

(Project INES-UBB 1440), of Chile, for partial funding of the densification aspect of this

project. Noemi Cruz is thankful for the support of Conacyt (Mexico) for master’s degree

scholarship.

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Article submitted: September 21, 2017; Peer review completed: December 16, 2017;

Revised version received and accepted: January 30, 2018; Published: February 2, 2018.

DOI: 10.15376/biores.13.2.2268-2282

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